Mass spectrometer



Aug. 19, 1958 J. ca f'sKlNNER ETAL MASS SPECTROMETER 2 Sheets-Sheet 1 Filed Nov. l0, 1955 INVENTORS.

J. G. SKINNER BY A, E. BUTCHER Hw@ www Aug. 19, 1958 Filed Nov. 10, 1955 J. G. sKlNNER ETAL ms sPEcTRoMETER 2 Sheets-Sheet 2 IN V EN TORS.

J. G. SKINNER A. E. BUTCHE-R FIG. 3.

A T TOR/VE KS United States atet MASS SPECTROMETER John G. Skinner, Corvallis, Oreg., and Alvin E. Butcher, Bartlesville, kla., assignors to Phillips Petroleum Company, a corporation of Delaware Application November 10, 1955, Serial No. 546,056

8 Claims. (Cl. Z50-41.9)

This invention relates to mass spectrometers. In another aspect it relates to improved grid mounting structure for vacuum tubes.

In recent years mass spectrometers have been developed from highly specialized academic research instruments for measuring the relative abundance of isotopes into analytical tools of extreme sensitivity and accuracy. At the present time, applications are being found for the use of mass spectrometers in process monitoring and control. Mass spectrometry comprises, in general, ionizing a sample of material under investigation and separating the resulting ions according to their masses to determine the relative abundance of ions of selected masses. The material to be analyzed usually is provided as a gas which is bombarded by a stream of electrons to produce the desired ions. Although both positive and negative ions may be formed by such electrical bombardment, most mass spectrometers make use of only the positive ions. These positive ions are accelerated out of the region of the electron beam by means of negative potentials. Such potentials impart equal kinetic energies to ions having like charges such that ions of different masses have different velocities after passing through the electrical field and, consequently, have different mornenta;

United States Patent 2,535,032 describes a mass spectrometer which is provided with two sets of three equally spaced accelerating grids. Direct potentials are applied to the outer two grids and a radio frequency potential is applied between the center grid and the outer two grids of each set. Ions which enter the space between the first two grids in proper phase receive maximum energy and are accelerated through the eld between the rst and second grids and the field between the second and third grids. The ions subsequently pass through a field-free drift space and enter the second group of three grids. The spacing between the grids, the frequency of the radio frequency voltage, and the magnitudes of the accelerating potentials are such that ions of predetermined mass receive suflicient energy to overcome a potential barrier and impinge upon a collector plate.

The mass spectrometer tube of the present invention is an improvement over the mass spectrometer disclosed in said Patent 2,535,032. The tube is provided a larger number of accelerating grids to define additional drift spaces. The grids are supported by annular plates having inner flanges integral therewith. The flanges support shield screens which extend between adjacent grids that dene the drift spaces. The plates are provided with a plurality of holes. Support rods extend longitudinally of the tube through spaced holes in the plates to align the plates. Electrical leads extend through other holes in the plates to connect the grids to external operating circuits. Insulating spacers separate adjacent plates.

Accordingly, it is an object of this invention to provide an improved mass spectrometer which operates upon the principle of energy selection of ions of a predetermined mass.

Another object is to provide improved means to position grids in vacuum tubes.

Other objects, advantages, and features of this invention should become apparent from the following detailed description taken in conjunction with the accompanying drawing in which:

Figure 1 is a schematic representation of a mass spectrometer incorporating features of this invention;

Figure 2 is a detailed elevation View of the tube of Figure 1; v

Figure 3 illustrates the grid support assembly; and

Figure 4 is a detailed view of the grid support plate.

Referring now to the drawing in' detail and to Figure 1 in particular, there is shownla mass spectrometer tube i 10 comprising a gas impermeable envelope, the interior of which is maintained at a reduced pressure by a vacuum pump, not shown, which communicates with the interior of tube 10 through a conduit 11. A sample of gas to be analyzed is directed into tube 10 through a conduit 12. An electron emitting filament 13 is disposed in one end of tube 10 and an ion collector plate 14 is disposed in the second end of the tube. The end terminals of lament 13 are connected to the respective end terminals of the secondary winding 15 of a transformer 16. The end terminals of the primary winding 17 of transformer 16 are connected to an alternating current source 18. The center tap of the secondary winding 15 of transformer 16 is connected to a negativelpotential terminal 19.

The gas sample supplied ,through conduit 12 diuses into an ionization chamber 22 in tube 10 which is dened by a pair of spaced grids 23 and 24 that are maintained at ground potential. Electrons emitted from filament 13 are accelerated'into chamber 22 by the potential difference between filament 13 and grids 23 and 24. A grid 26 is positioned between lament 13 and grid 23. Grid 26 is connected to the output of an emission regulator circuit 27, which can be of the type disclosed in the copending application of M. C. Burk, Serial No. 412,790, filed February 26, 1954. The input of emission regulator 27 is connected to the center tap of transformer windingV 15. This emission regulator is provided for the purpose of applying a potential to grid 26 of magnitude such as to maintain a constant ow of electrons into ionization chamber 22 irrespective of minor fluctuations in the electron emission from filament 13. In this manner the rate at which gas molecules are ionized in chamber 22 by electron bombardment is a function of only the gas pressure in the chamber.

A first collimating electrode 28 is-positioned on the second side of ionization chamber 22 and is connected to the contacter of a potentiometer 20. One end terminal of potentiometer 20 is connected to a negative potential terminal 21, the second end terminal of potentiometer 20 being grounded. A second collimating electrode 28'is `spaced from electrode 28. Electrode 28' is connected to thecontactor of a potentiometer 20. The end terminals of potentiometer 20 are connected to terminal 2l and ground, respectively.

The positive ions produced in ionization chamber 22 are accelerated by the negative potentials applied to electrodes 28 and 28' so as to travel through the tubev toward collector plate 14. A first set of three equally spaced grids 35, 36 and 37 is positioned in tube 10 between grid 28 and collector plate 14; a second set of equally spaced grids 38, 39 and 40 is positioned in spaced relation with the rst set ofl grids; a third set of equally spaced grids 42, 43, and 44 is positioned in spaced relation with the second set Vof grids; a fourth set of equally spaced grids 45, 46 and 47 is positioned in spaced relation with the third set of grids; and a fifth set of equally spaced grids 49, 50 and vSlis positioned in spaced -re- 'J lation with the fourth set of grids. Grid 35 is connected to the contactor of a potentiometer 29. One end terminal of potentiometer 29 is connected to a negative potential terminal 30, the second end terminal1ofpotentiometer 29'b'eing connected to ground. The contactor of potentiometerl29 canbe adjusted fto apply selected negative accelerating potentials to grid 35.h Grids 36, `39,` 43, 46 and 50"are connected to one another and to the output terminal of an electronicV switch 55'; The inputl terminal of switch 55 is connected to one output terminal of a radio frequency oscillator 56. Switch 55 is con-` trolled by theoutputV of asquare wave generator 57, whichin'trn is energi'zedby th'output of an audio oscillator 58. Switch55 is thus turned on and-olf at the' frequency of oscillator' 58`toapply"`th`e output of oscillator"56`to`grids`36, 39, 43, 46 and50. Grid 35 is connected to one output terminal of Aa'ste'p back detector 61, which in turn is Aconnected to anoutputr terminal of oscillator 56. The secondou'tp'ut terminal of detector 61 is connected to grid 51. Grids '35 and 51 'are connected to one another by a voltagedividingnctwork 63 which comprises tive resistors 64, 65, 66, 67 and 68' that are connected in series relation. Grids `37 and 38 are connected to one another and to the junction between resistors 64 and 65;V grids 40 and 42 are connected to one another and to the junction between resistors 65 and 66; grids 44 and 45 are connected to one another and to the junction between resistors 66 and 67; and grids 47 and 49 are connected to one another and to the junction between resistors 67 and 68.

A plurality of closely spaced"stopping grids 70 is positioned between grid 51'and' collector plate 14. Grids 70 are connected to one another and to the output of a stopping detector 71, which in turn is connected to an output of oscillator 56. A plurality. of suppressor grids 72 is positioned between grids 70 and collector plate 14. Grids 72 are connected to a negative potential terminal 73 to suppress secondary electrons which may result from ions impinging upon metal parts of the tube. A grounded shield 75 is positioned adjacent collector 'plate 14.

Collector plate 14 is connected to one input terminal of an amplifier 76 which is tuned to pass signals of the same frequency as the frequency of oscillator 58. The second input terminal of amplilier 76 is connected to ground. The output terminals' of amplifier 76 arel con nected to first input terminals of a phase detector 77. The second input terminals of phase detector 77 are connected to output terminals of oscillator 58. The output terminals of phase detector 77 are connected to a suitable indicating means such as a recorder 78.

The circuits illustrated schematically in Figure 1 are described in detail in the copending application of M. C. Burk and F. W. Karasek, Serial No. 480,696, tiled January l0, 1955.

In the mass spectrometer tube of Figure 1, the spacings s between grids 35 and`36, 36 and 37, 38 and 39, 39 and 40, 42 and 43, 43and 44, 45 and 46, 46 and 47, 49 and 50, and 50 and 51 are maintained equal. The spacings r between the `centers of grids 37 and 38, 40 and 42, 44 and 45, and 47 and 49 can be represented by the expression:

where n is an integral number, all dimensions being in inches. In one embodiment of this invention, s was 0.118 inch and the four ns, proceeding from lament 13 to collector plate, were five', nine, four and seven, respectively. The values of n can be varied, they may be equal, and more or fewer drift' spaces can be provided, if desired.

Tube is illustrated in detailin Figures 2, 3 and 4. Each of the grids is formedof a circular screen `which is supported by an annular plate, such a's'38a shown in Figure 4 inconjunction- With=grid 38. llate'38asisis provided an improved mass spectrometer tube.

provided with an inner flange 38b and an outer flange 38e, the latter providing rigidity. Plate 38a is also provided with a plurality of holes 38d through which support rods and electrical leads pass. Grid 38 is at tached to plate 38a by means of a ring 38e which is welded to plate 38a. Grid 38 is thus sandwiched between ring 38e and plate 38a. Grid 38 preferably is formed of tungsten wire and plate 38a is formed of Monel metal. Each of the other grids in tube 10 is supported by a corresponding annular plate designated by an a reference numeral.

The several annular plates are positioned with respect to one another by support rod assemblies 85, 86, 87 and 88 which extend through corresponding holes in the plates.

Assembly 85, for example, comprises a rigid metal rod a having an insulating sleeve 85b attached to one end thereof by a clamp 85e. Sleeve 85b extends nearly the entire length of rod 85a and the outer diameter thereof is approximately the same as the diameter of the holes in the annular screen mounting plates. An insulating spacer sleeve 85d extends between clamp 85C and plate 26a. Corresponding insulating spacer sleeves 85e, 85f, 85g, 85h, 851, 85j, 85k, 851, 85m, 8511, 85o, 85p, 85g, 85r, 85s, 85t, 85u and 85V, are mounted on sleeve 85b between respective pairs of plates 26a, 23a; 24a, 28a; 28a', 35a; 35a, 36a; 36a, 37a; 38a, 39a; 39a, 40a; 42a, 43a; 43a, 44a; 45a, 46a, 46a, 47a; 49a, 50a; 50a, 51a; 51a, 70a; 70a, 70a; 70a, 72a; and 72a, 72a. Metal spacer sleeves 90, 92, 93, 94 and 95 are mounted on sleeve 85b between respective pairs of plates 23a, 24a; 37a, 38a; 40a, 42a; 44a, 45a; and 47a, 49a. An insulating sleeve 96 extends between plate 72a and a guide plate 97. An insulating sleeve 91 extends between plates 28a and 28a'. An insulating sleeve 98 and a spring 99 extend between plate 97 and a clamp 101 on the second end of rod 85a. Spring-99 retains the plates tight against the spacers. Springs 102 and,103Y are ,mounted on respective plates 26a and 97 to align the assembly in tube 10. The assembly is inserted lin tube 10 and end 104 is fused to the tube.

Cylindrical screens 105, 106, 107 and 108 surround the four drift spaces. Considering Figure 4, for example, screen i is sandwiched between ange 38E-'and a cylindrical band 38f. Screens 105, 106, 107 and 108 serve to form uniform drift spaces down the center of tube 10.

The electrical leads extending to the several grids in tube 10 pass `through corresponding holes in the annular plates, as illustrated in detail in Figure 3.

The collector plate assembly and the sample inlet tube evacuation system are illustrated in detail in the copending application of I. G. Skinner, Serial No. 546,- 221, tiled November 10, 1955.

From the foregoing description of a preferred embodiment of this invention it should be apparent that there The individual grids which control the passage of ions to the collector plate are positioned by novel annular mounting plates which also support cylindrical guide grids. The several grid mounting plates are positioned by support rods which extend through the mounting plates.

While the invention has been described in conjunction with a present preferred embodiment, it should be evident that it is not limited thereto.

What is claimed is:

l. A mass spectrometer tube comprising a gas impermeable envelope enclosing means to produce ions of a sample to be analyzed; a collector plate spaced from said means to produce ions; and a plurality of grids positioned between said means to produce ions and said col lector plate, said grids each comprising a at annular metal plate having a rst flange at the inner edge and a second ilange at the outer edge, a metal screen extending across the opening in said plate on the side thereof opposite said fir/st ilange, and a second annular plate secured to said at plate, the periphery of said screen being positioned between said plates.

2. The combination in accordance with claim 1 wherein each of said ilat plates has a plurality of openings therein, a plurality of insulating support rods extending through respective ones of the openings, and hollow spacers on said rods between said flat plates to separate said flat plates.

3. The combination in accordance with claim 2 wherein the lirst flanges of two adjacent ones of said flat plates oppose one another, and further comprising a cylindrical screen extending -between said two plates, said screen engaging the opposing first anges of said two plates, and annular bands securing the ends of said cylindrical screen to said first flanges.

4. Apparatus for controlling the passage of charged particles comprising a at annular plate having a first flange at the inner edge and a second flange at the outer edge, a metal screen extending across the opening in said plate, the diameter of said screen being greater than the inner diameter of said plate, a second annular plate of inner diameter substantially equal the inner diameter of said :dat plate, and means securing said second plate to said flat plate on the side thereof opposite said inner flange, the edge of said screen being positioned between said plates.

5. Apparatus for controlling the passage of charged particles comprising a pair of grid assemblies, each of said assemblies comprising a at annular plate having a iirst flange at the inner edge and a second flange at the outer edge, -a metal screen extending across the opening in said plate, the diameter of said screen being greater than the inner diameter of said plate, a second annular plate of inner diameter substantially equal the inner diameter of said at plate, and means securing said second plate to said flat plate on the side thereof opposite said inner ange, the edge of said screen being positioned between said plates; means securing the two flat plates in spaced relationship so that the lirst flanges oppose one another; a cylindrical screen extending between said at plates; and annular bands securing the ends of said cylindrical screen to said first anges.

6. A mass spectrometer comprising a gas impermeable envelope enclosing means to ionize a sample of material to be analyzed; a collector plate spaced from said means to ionize; a plurality of groups of grids spaced in a line between sa-id means to ionize and said collector plate, each of said groups comprising three grids in spaced relationship with one another, the spacings between adjacent grids being equal, the spacings between adjacent groups of said grids being substantially Il 24m-1) Where n is an integral number and s is the spacing between adjacent grids in each group, a second grid po- Sitioned between said collector plate and said groups of grids, said grids each comprising a at annular metal plate having a first flange at the inner edge and a second flange at the outer edge, a metal screen extending across the opening in said plate on the side thereof opposite said first iiange, and a second annular plate secured to said flat plate, the periphery of said screen being positioned between said plates; means applying steady potentials to the two end grids in each of said groups of grids; means applying a potential to said second grid of polarity opposite the polarity of the ions being detected; means applying an alternating potential to the center grid in each of said groups of grids; and detecting means connected to said collector plate.

7. The combination in accordance with claim 6 wherein each of said iiat plates has a plurality of openings therein, a plurality of insulating support rods extending through respective ones of the openings, and hollow spacers on said rods between said dat plates to separate said at plates.

8. The combination in accordance with claim 7 wherein the first lianges of the end at plates in each of said groups extend Atoward an adjacent group, and further comprising a cylindrical screen extending between each of said groups, said screens engaging the first anges of said flat plates, and annular bands securing the ends of said screens to the first flanges.

References Cited in the file of this patent Bennett: Journal of Applied Physics, vol. 2l, Feb. 1950, pp. 143-149 relied on. (Copy in Div. 44.) 

4. APPARATUS FOR CONTROLLING THE PASSAGE OF CHARGED PARTICLES COMPRISING A FLAT ANNULAR PLATE HAVING A FIRST FLANGE AT THE INNER EDGE AND A SECOND FLANGE AT THE OUTER EDGE, A METAL SCREEN EXTENDING ACROSS THE OPENING IN SAID PLATE, THE DIAMETER OF SAID SCREEN BEING GREATER THAN THE INNER DIAMETER OF SAID PLATE, A SECOND ANNULAR PLATE OF INNER DIAMETER SUBSTANTIALLY EQUAL THE INNER DIAMETER OF SAID FLAT PLATE, AND MEANS SECURING SAID SECOND PLATE TO SAID FLAT PLATE ON THE SIDE THEREOF OPPOSITE SAID INNER FLANGE, THE EDGE OF SAID SCREEN BEING POSITIONED BETWEEN SAID PLATES. 